The sophistication of internal combustion engines and engine operating schemes continues to increase, and innovations in design and operation are revealed almost daily. One driving force behind many of the design changes in recent years has been increasingly stringent emissions requirements. One general approach to improving emissions quality relates to treatment of combustion products downstream from the engine. In other words, exhaust gases produced by the engine are treated via a variety of chemical and/or physical processes in an attempt to remove or reduce undesired constituents. Other engine developers have focused more on the combustion process itself. Manipulation of fuel injection quantity, frequency, timing and even the type of spray pattern has been shown to have various effects on engine emissions. Of particular interest to engineers are the increasingly stringent government requirements relating to emission limits on various nitrogen-oxygen compounds, known collectively as “NOx”.
It has been discovered that enhancing mixing of air and fuel prior to ignition in an internal combustion engine cylinder can help reduce NOx levels in the engine exhaust. One approach in particular is known in the art as “homogeneous charge” ignition. In compression ignition engines, this approach is widely referred to as “HCCI”. In a homogeneous charge mode, fuel may be injected into a compression ignition engine cylinder prior to the point during an engine cycle at which cylinder conditions will trigger autoignition. This differs from a more traditional approach, wherein fuel is primarily injected during an engine cycle close to a point at which autoignition can occur. In other words, rather than fuel more or less continuously combusting as it leaves the fuel injector tip, in homogeneous charge mode the fuel may be injected in advance of autoignition conditions, such that the fuel and air have relatively more time to mix as the piston travels upward in the cylinder.
Homogeneous charge operation tends to be relatively sensitive to various operating conditions external to and internal of the engine. Ambient temperature and pressure, as well as the timing of autoignition conditions in the engine cycle, for example, can affect the ability of an engine to successfully operate in a homogeneous charge mode. Another factor bearing on the success of operation relates to limitations of homogeneous charge engines to only a portion of their theoretical load range. Increases in power demand on an engine are typically accommodated by increasing the quantity of fuel combusted in a given engine cycle. Where an engine is operated in homogenous charge mode, injected fuel tends to ignite more uniformly, in many cases generating relatively greater cylinder pressure and cylinder pressure spikes than conventional ignition of an equivalent fuel quantity. As the power demands and thus injected fuel quantity increase, the engine may reach a point at which physical stresses on the engine hardware may be too great to accommodate a larger homogenous charge combustion event.
U.S. Pat. No. 6,725,838 to Shafer et al. sets forth one fuel injector having dual mode capabilities, for operation in a homogeneous charge mode, a conventional mode or a mixed mode. While Shafer discusses various designs and operating strategies, there is always room for improvement.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.